The skeletal muscle fibers are long and cylindrical, with rounded ends. They extend the full length of short muscles but only half way along larger ones. Each one of these fibers is a cell, called myocyte, which contains many nuclei.; thus, it is said that they are multinucleated. The relative long nuclei occupy a peripheral position just under the cell surface.
The cytoplasm of the one cell (a skeletal muscle fiber) is called sarcoplasm, which contains striated cylindrical elements called myofibrils. The myofibrils extend the full length of the fiber. The pattern of striation on these thread-like components shows precise lateral registration. Thus, each so-called band of the muscle fiber consists of closely approximated segments of numerous myofibrils. Each myofibril in turn contains sarcomeres, which are the contractile segments of the myofibril. A sarcomere is made up of actin and myosin filaments, which are motility proteins.
Seen under a high power microscope in longitudinal section, the skeletal muscle fibers show a distinctive pattern of alternating dark and light-staining transverse bands. Under polarized light, the dark-staining bands are anisotropic, which means they double-refract light (in different directions), whereas the light-staining bands are isotropic (identical in all directions). Therefore, the dark bands are called A bands (for anisotropic), while the light bands are called I bands (for isotropic). A dark line, called Z line, bisects each band. In relaxed fibers, a paler region termed the H zone can sometimes be distinguished. Although apparently each band traverses the entire muscle fiber in a continuous way, in reality, this is not the case.
Below, a schematic picture which shows the location of a skeletal muscle fiber and the myofibrils it is composed of in relation to a fasciculus (fascicle) and a muscle.
